Fluids based on fluorocarbon compounds are widely used in systems for heat transfer by vapor compression, notably devices for air conditioning, heat pumps, refrigeration or freezing. These devices have in common that they are based on a thermodynamic cycle comprising vaporization of the fluid at low pressure (in which the fluid absorbs heat); compression of the vaporized fluid to a high pressure; condensation of the vaporized fluid to liquid at high pressure (in which the fluid gives up heat); and expansion of the fluid, ending the cycle.
The choice of a heat transfer fluid (which may be a pure compound or a mixture of compounds) is dictated on the one hand by the thermodynamic properties of the fluid, and on the other hand by additional constraints. Thus, a particularly important criterion is the effect of the fluid in question on the environment. In particular, chlorinated compounds (chlorofluorocarbons and hydrochlorofluorocarbons) have the disadvantage of damaging the ozone layer. Henceforth, nonchlorinated compounds such as hydrofluorocarbons, fluoroethers and fluoroolefins are therefore generally preferred.
Another environmental constraint is the global warming potential (GWP). It is therefore essential to develop heat transfer compositions having a GWP as low as possible and having good energy performance.
Moreover, to lubricate the moving parts of the compressor (or compressors) of a vapor compression system, a lubricating oil must be added to the heat transfer fluid. The oil may generally be mineral or synthetic.
The lubricating oil is selected based on the type of compressor, and such that it does not react with the heat transfer fluid proper and with the other compounds present in the system.
For certain heat transfer systems (notably of small size), the lubricating oil is generally allowed to circulate in the whole circuit, the pipework being designed in such a way that the oil can flow by gravity to the compressor. In other heat transfer systems (notably of large size), an oil separator is provided immediately after the compressor as well as an oil level management device, ensuring return of the oil to the compressor or compressors. Even when an oil separator is present, the system pipework must also be designed so that the oil can return by gravity to the oil separator or to the compressor.
U.S. Pat. Nos. 7,279,451 and 7,534,366 describe compositions based on fluoroolefins, for example based on tetrafluoropropene. Various lubricating oils are also envisaged. Moreover, the document reports the results for compatibility between 1,2,3,3,3-pentafluoropropene (HFO-1225ye), 1,3,3,3-tetrafluoropropene (HFO-1234ze) or 3,3,3-trifluoropropene (HFO-1243zf) and lubricating oils of the polyalkylene glycol type.
When the heat transfer compound or compounds have poor miscibility with the lubricating oil, the latter has a tendency to be trapped at the level of the evaporator and not return to the compressor, which does not allow the system to function correctly.
In this respect, there is still a need to develop heat transfer compositions with low GWP (and displaying good energy performance), in which the heat transfer compounds have good miscibility with the lubricating oil.
In particular, 2,3,3,3-tetrafluoropropene (HFO-1234yf) is a heat transfer compound that is particularly interesting notably owing to its low GWP and its good energy performance. However, its miscibility with certain lubricating oils is imperfect and limits its application. It is therefore desirable to improve the miscibility of compositions based on HFO-1234yf with the usual lubricating oils.